Tone control



Jn.21, 1936. H. M.*| Ew|s 2,028,511

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INVENT'oR HAROLD MILLER LEwls BY QW,.W,WMJ

4ATroRwsYs Jan. 2l, 1936. H. M.l LEWIS TONE CONTROL Filed Aug. 2o,4 1932 oRNEYs l HAROLD MILLER LEWIS NMJQV Patented Jan. 21, 193e UNiTEo STATES PAT-ENT ori-ice 'roNE coN'rRoL Harold Miller Lewis, Douglaston, N. Y., assigner Hazeltine Corporation Application August 2o, 1932, serial No. '629,604

s o1aims. (ci. 25o-2o) This invention relates to sound-reproducing systems,land more particularly to means vfor automatically varying the tone of the reproduced sound. I

In accordance with this invention, the tone of the reproduced sound, as, for instance.l in an automatic volume control radio receiver, is varied automatically in such a manner that the high frequencies, are reduced increasingly as the ampliiication is increased.V

In a radio receivenwheneverthe sensitivity is maximum, annoying hissing sounds `of the higher audio frequencies are heard. With an automatic volume control receiver, whenever the amplification is automatically increased, as, for instance, when tuning on either side of a broadcast signal, when tuning from station to station,

or when receiving a weak signal, the hissing sounds are quitey objectionable. Furthermore, the static and noises of high audio frequencies, which are reproduced when the receiver is operating at maximum sensitivity and not tuned to any signal, are also quite objectionable.

In sound reproduction, as from a talking movie sound record, the passage ofthe record through thereproducer during the silent sequences with the amplier set for maximum amplication resuits in the production of a disagreeable highv'taken in connection frequency hiss or scratch frequency sound.

It is the primary object of this invention to reduce the audio-frequency response to the higher frequencies whenever the sensitivity of the amplier system is such as to produce vthese high audio frequencies to an objectionable extent.

It is a particular object of this invention to provide a radio receiver having an automatic volurne control arrangement and an automatic tone control arrangement, each controlled by the intensity of the received carrier wave, so that whenever the' sensitivity of the receiver is greatly increased clue to reception of signals of low intensity, the. high frequencies in the output are correspondingly reduced; or, alternatively, whenever the receiver is tuned to powerful signals resulting in a decrease of sensitivity, the proportion of high frequencies to. low frequencies is in creased.

These and further objects ofthe invention willbecome apparent from the following description with the accompanying drawings.

In accordance with this invention, means are provided for rectifying the received carrier wave, preferably after it has been amplified, to produce a source of direct-current voltage which is proportional to the received carrier wave. This voltage is then utilized to control the characteristics of a tone control so that the attenuation of the higher of the'detected audio frequencies is varied in a predetermined manner as a function of the 5 control voltage.

Where the receiver is of the superheterodyne type, the direct-current control voltage will preferably be obtained by rectifying the intermediatefrequency carrier rather than the original car.- lo rior-frequency current. In either case, however, y the direct-current control voltage obtained is proportional -to the received carrier wave amplitude, and its proportionality depends upon the type of detection used. For example, with linear detection, which is the type preferred, the directcurrent voltage is almost directly proportional to'v the received carrier wave amplitude.

Although `separate detectors may be used for `providing the various control voltages, a single o most simple arrangement contemplated is the one 3g in which resistance alone is varied, although a variety of circuits variable with voltage to control the frequency impedance characteristics of the tone control will occur to those skilled in the art.

In its preferred embodiment, this invention Y provides. as the variable resistance elemento! the tone control, a thermionic vacuum tube so arra-ngedthat the directcurrent control voltage may be applied between the control grid and the .4,0 I.cathode to vary the plate-cathode resistance. If

thecontrol voltage is connected to make the grid more negative with respect to the cathode asvthe control voltage increases, the resistance of the tube will increase when the control voltage increases. A xed capacity is connected in series with the plate of the vacuum tube as a shunt across the audio-frequency transmission system. With this arrangement, when no signal or a very weak signal is being received,` the direct-current control voltage is low and the resistance of the vacuum tube is low, thereby providing a shunt .path consisting of capacity and resistance so proportioned as to by-pass the higher audio frequencies toy whatever .degree may be desired. 45g;V

When a strong signal is being received, the dic rect-current control voltage is large, causing the resistance or the vacuum tube to become high, and thereby practically removing the shunt tone control and permitting as perfect delity of re- 'I Fig. 1 is a schematic diagram of a radio receiver employing the automatic tone control of thisinvention; v

Fig. 2a is a diagram of a manual tone control arrangement;

Fig. Zbis a diagram of a simple type of automatic tone control arrangement corresponding to the type shown in Fig. 2a;

Fig. 3 vrepresents asuperheterodyne radio receiver employing both automatic volume control and automatic tone control and utilizing a single detector for producing the audio frequencies and direct-current control voltages; 5

Fig. 4 is a diagram giving the circuit arrangement 'of the receiver shown in Fig. 3;

Fig. 5 is a circuit diagram showing a superheterodyne receiver employing a separate dates- A tor for controlling the action of the tone control.

, the tone control circuit I1, which will be describedl In Fig. i, which is a schematic diagram of a radio receiver constructed accordance with this invention, the antenna I0 and ground Ii are connected to the radio-frequency ampliiler I2, the output of which is connected to the detector i8. The detected output is connected through hereinafter, to the audio-frequency amplier M,

.by whiohthe detected signals are amplified to be reproduced by the loud speaker I5.. A second detector I, for controlling the automatic tone control Il, is connected in parallel with the rst detector I3.

In operation, the modulated radio-frequency signal currents are collected by the Lantenna.- ground system IEI--II and applied to the radiofrequency amplifier I2. The output of the. am-

pliiler I2 is applied to the two detectors i3 and I6. The detector I3 is arranged to produce the audio-frequency currents and supply them through the tone control I'I to the audio-frequency amplifier It. The output of the audiorequency amplier It is connected to the loud speaker I5. The tone control i1 controls the fldelity of the audio frequencies applied to I5 and has a frequency-attenuation characteristic, which is, as will be explained hereinafter, dependent upon the value of a direct-current potential applied as a control voltage from the detector i8.

The 'detector I@ is supplied with radio-frequency energy from the radio-frequency 'ampliner i2. This detector is adjusted to yield a steady value of direct-.current voltage which is proportional to the amplitude of the carrier wave applied to its input. It will thus be clear that when a strong signal is being received, the direct-current voltage output of the detector te is large, and hence thefrequency-attenuation characteristic' of the tone control I1 is die'rent from what it will be when a weak signal is received and the directcurrent voltage output of the detector la is small. Thus, by virtue of the elements It and I1, the quality ofthe signals delivered to the loud vspeaker l5 is made dependent upon the strength of the received signals.

In Fig. 2a the conductors 22-2Pi and 23-25 carry an audio-frequency current similar to that carried by the conductors between the `detector I3 and the audio-frequency amplier I4 of Fig. l. The circuit in this figure shows a network which is similar to the manual tone control commonly employed in the present-day radio receiver for variably attenuating high audio frequencies. Tins network, which is connected between the conductors 22-24 and 23-25, comprises a condenser 26 and a tapped resistor 2l. The portion vof the tapped resistor 21 which is connected in series with the condenser 2S between the conductors 22-26 and 23-25' is selected by means of the tap switch 2B. When the switch 28 is adjustedto include all of the resistor 21 in series with the condenser 26, the shunt path, comprising condenser 26 and resistor 21, has practically no eiect, and the signals applied to the terminals 22 and 23 are freely transmitted to the terminals 2a and 25 without attenuation. When, however, the switch 28 is adjusted to include only a small portion o! the resistor 21, a shunt path is provided which attenuates the voltage at the higher audio frequencies applied to the terminals 22 and 23 more than it does that at the lower frequencies, depending upon the values of the condenser 26 and the portion of the resistor 21 then included in the shunt path. I

Fig. 2b illustrates the circuit arrangement of the tone control lI-of Fig. l. In this circuit a shunt mth, comprising the condenser 2S and the impedance of the tube 2d, is provided between the conductors 22-26 and *2t-25, which connect the output of the detectorl i3 with the input of the audio-frequency ampliiier It. The frequency-attenuation characteristic of this shunt path is controiled by a variation of the grid Avoltage of the tube 3a. The grid voltage is determined by the output of the detector It of Fig. 1, as will be explained hereinafter, and when the potential applied to the terminals 3l and 32 is high and of is low, the resistance of the plate-cathode path is low, and the higher frequencies are correspondinsly attenuated.

The plate electrode and cathode of the tube 29 are supplied with appropriate operating potentials from sources which are not shown. The impedance 3@ is included between the plate-potential source and the plate electrode of the tube i@ in-orderto make the impedance of this parallel path from condenser 26 to the line 28-25 via the plate power supply higher than the impedance o the plate-cathode path of the vacuum tube, in order that the plate-cathode resistance may control the shunting ofthe high audio-frequency currents. A choke or any. other ilter offering a high impedance to all audio frequencies may be substituted for the resistance 30.

n connecting the tone control shown in 2b in the circuit at I1 of Fig. 1, direct-current voltage output of the detector It is applied'to the terminals Sl-S and so poled as to make terminal 3i negative relative to terminal 82. Thus. when a strong signal is received and a large direct-'current voltage is developed in the output of the detector i8, the resistance of the tube 28 is high, and the audio-frequency signals developed in the output of vthe detector I3 pass without attenuation to the audio-frequency amplifier Il.

er, is a preferred form. The reason for this prefl erence is that when weak signals are received, the

receiver is generally operated at maximum sensitivity, and more static and extraneous noises are present, and their disturbing sound can be reduced by attenuating the higher audio frequenciesv lmore than the lower audio frequencies. I'his is particularly -true when the receiver is equipped also with an automatic volume control, and the sensitivityis consequently automatically increased when weak signals or no signals at all are being received. y

The arrangement shown in Fig. 3 illustrates a superheterodyne receiver employing an automatic volume control. In this iigure, the parts similar to those shown in Figs. l and 2 are similarly designated. The output of the radio-frequency amplifier I2 is supplied to a modulator or first detector I9, which has also supplied thereto highfrequency oscillations produced bythe oscillator 2l for the purpose of combining with the received radio-frequency signals and producing the intermediate-frequency signal currents, which latter currents are further amplified by an intermediate-frequency amplifier 2d. The output of the intermediate-frequency amplifier 20 is supplied to the second detector I3'. The detector I3' takes the place of the two detectors I3 and l@ of Fig. l, and its output is supplied to the tone control Il, which is similar to that of Fig. l, and to the filter I8, the output of which controls the characteristics of the automatic tone control Il, as well as the amplification of the radio-frequency amplifler I2 and the intermediate-frequency amplier 23. The rcurrents passed through the automatic tone control Il ar amplifledioy the audio-frequency amplifier I4 and reproduced-by the loud speaker I 5. 'I'he oscillator 2| supplies to the modulator I9 energy of a frequency differing from the incoming signals of the carrier frequency by the intermediate frequency to which the intermediate-frequency amplifier is tuned. Thus, the modulator I9 supplies to the intermediate-frequency amplifier 20 the converted intermediate carrier and signal side hands representative of the audio frequencies being transmitted. The specific arrangements of the partsof the circuit just referred to include the usual superheterodyne arrangements and need not be further described.

The detector I3' is adjusted to develop the tion to the amplitude of the signal being received.

The sensitivity ofthe, ijecelver is reduced when the incoming signals are strong, but is increased when the incoming signals are weak.

In accordance, then, with this well-known method, the direct-current voltage output of the iilter I8 is shown as being applied tothe radiovfrequency amplifier I2 and the intermediatefrequency ampliiier 2B to conti-ql the gain of these amplifiers and hence provide automatic volume control. At the same time, however, the direct-current output of the filter I8 is applied to the tone control unit I'I to control its attenuation characteristics so that when weak signals are received, the higher audio frequencies are correspondingly attenuated. In this figure the tonecontrol arrangement is assumed to' be of the type shown in Mg. 2b.

The resulting attenuation of the higher audio frequencies, when receiving weak incoming carrier-frequency currents, is particularly desirable where the receiver has the automatic volume control illustrated. As is well known, when a radio receiver is equipped with an automatic volume control, the Sensitivity of the receiver, which is s low when receiving signals from a strong local station, increases rapidly as the receiveris tuned off resonance with the frequency of this station, and an undesirable hissing sound is heard on either side of the resonance point for every signai tuned through in operating the receiver. When, however, an automatic tone control is employed according to this invention, as in the system of Fig. 3, these high-frequency hissing sounds, oc

I casioned whenever a strong carrier frequency is desired audio-frequency signal currents and to supply them through thetone control Il to the audio-frequency amplifier I l. It also develops a direct-current voltage vproportional to the received carrier wave and supplies it to the filter I8. -The filter Il is for the purpose of removing any audio or other frequency components from thevoutput of the detector in order that the variable direct-current voltage shall be proportional to the amplitude of the carrier wave received. As is wellknown in the artof automatic volume control, the amplifying action of the vacuumtubes, composing the radio-frequency and internot present4 in the receiver, are reduced by the automatic attenuation of the higher audio frequencies.

Thus, ,the automatic tone control, in addition to its merits when operated in conjunction with a simple receiver as shown in Fig. 1, enhances v and perfects the action of an automatic volume control receiver. Since the form of tone control of this invention is a potential-operated device and utilizes the direct-current potential already provided in receivers equipped with automatic volume control; very little more than the unit shown in Eig. 2b is required to be added in order to provide the automaticv tone control.

In both Fig. 1 and Fig. 2 the tone control unit I1 has been shown Vas interposed in the line between' the detector and the audio-frequency ampliiier. It will be clear, however, that the tone control might equally wellbe inserted between the audio-frequency amplifier I4 and the loud speaker I5, or at yany appropriate point in an ampllfylngystem.

4 represents diagrammatically a superheterodyneradio receiver similar to that'show'n audio-frequency ampliilerare shown in detail.,

The signal received by the antenna-ground system Ill-II and amplied by the radio-frequency amplifier I2 is converted into intermeand. 38, while currents of high or intermediate -radio' frequency are by-passed to ground bythe condenser 31. The connections d@ and 4I supply --a direct-current potential, proportional to the' received carrier wave, to the control grids of the vacuum tubes in the ampliers I2 and 2li, respectively. 'Ihis permits the amplication of these vamplifiers to4 be automatically contr-olledto give substantially uniform-output regardless of the signal input intensity. Resistors 44 and 45 lserve as isolators to prevent audio and radio-frequency components present from feeding back into the circuits of units I2 and 20.

The connection 42 supplies the audio-frequency voltages developed by the detector i3' to the audio-frequency amplifier I4, where it is ampliied and applied to the loud speaker I5. The

first vacuum tube stage of the ampliiier I4 is shown in detail and includes the vacuum tube 5u having input circuit elements, including the isolating resistor 45 and condenser 48, for preventing high-frequency and direct current being impressed upon the input of the amplifier tube 50, the volume control resistor 49,. and biasing battery 52. The biasing battery 52 is by-passed by the condenser 5I connected between the end of resistor 49 and the cathode of the tube 50. Any

desired amount of the voltage fluctuations existing across the volume control resistor 49 may be impressed upon the grid of the ampliner tube 59 by means of the variable contact 49'. The output of the amplifier tube 50 includes the primary of the output transformer 53, the resistor 56, and the high-potential source 54. The resistor 56 and the high-potential battery 54 are by-passed for audio-frequency currents by the condenser 51.

'The audio-frequency outputA of the amplier tube 50 is suppliedl to the remaining part of the audio-frequency amplifier, represented at 58, by means of the transformer 53. The output ofthe amplifier 58 is reproduced by the loud speaker I5.

In shunt with the output Vpath just described is included the tone control tube 29 and the condenser 26 constituting a shunt path between the plate and cathode of the amplifier tube 50.. The tone control tube 29is similar to that shown in Fig. 2b and has its grid connected to the grid ends of resistors 38 and 39 by means of the connection 43,which latter includes a lter comprising the resistor 41 and condenser 41'. This provides a variable tone control arrangement in accordance .with the present invention.

Battery 54 supplies the plate potentials for the tubes 29 and 50. The connection between'the plate of the tube 50 and the battery 54 is throughy the primary of the out-put transformer 53 and the resistor 56. By-pass condenser 51 is con-l nected to the junction of the primary of transformer 53 and resistor 56 and to ground to bypass high-frequency currents. The plate supply to the tube 29 is through the resistor 39, and the portion of the battery 54 included in the plate circuit oi tube 29 is by-passed for high-frequency currents by means of the condenser 53. The resistance of the resistor 39, included in the plate circuit of the tone control tube 29, is chosen to have a high impedance as compared with the plate-cathode resistance of the tube 29.

It will be clear that when the grid of the tube 29 becomes increasingly negative with respect to its cathode, as when the direct-current potential developed across the resistor 39 and applied to the grid of 29 through the connectioni and the resistor t1 increases, the plate-cathode path oi the tube 29. increases in resistance and the audio frequencies in the' outputl of tubell are transmitted` to the amplier 58 with'maximum fidelity. When little or no direct-current bias potential from the resistor 39v is applied to the grid of the tone control tube 29, the plate-cathode resista'nce of vthe tube 291s low,and the higher laudio frequencies are readily -by-passed in the output of the amplier tube 59 through the condenser 26 and the tone control tube 29.

From the foregoing description it i: clear that the circuit just described provides both automatic tone control and automatic volume control; that is, when no signal or a very weak signal is being received, little or no audio-frequency voltages exist across the resistors 38 and 39, and little or no direct-current potential is developed there. Hence no bias is furnished to the amplier units I2 and 2u, so that they function at maximum sensitivity. Furthermore, no bias is furnished to the tone control tube 29, so that the high frequencies are readily by-passed through the tone control shunt circuit, and the high-frequency noise and'static existing in the output and reproduced by the loud speaker I5 are reduced. With i a strong incoming signal, direct-current potentials developed across the resistors 39 and 39, as supplied to the amplifier units I2 and 20, reduce the sensitivity of the receiver, and at the same time the grid of the tone control tube 29 is biased more negative with respect to its cathode, thereby increasing the impedance of the tone control path, and permitting all of the audio-frequency currents to pass from the audio amplier to the amplifier 5B to be reproduced by the loud speaker I5 with substantially maximum delity.

Included in shunt with the output and tubecontrolled tone control path of the amplifier 50 is a manual tone controlcircuit including the condenser 26' and the variable resistance 29'. By means of the resistor 29' the impedance of this path may be so adjusted that the proportion of high frequencies by-passed in the output ot the ampli'er tube 33 may be increased or decreased so that the tone of the signals as reproduced by the loud speaker I5 may be adjusted to suit the listeners taste regardless of the sensitlvity of the receiver.

In connection with the circuit of Fig. 4, it should be noted that the-several isolating resistors 44, 45, and 41, together with condenser 41 and by-passingcondensers which would normally b e included in the amplier units I2 and 20. as is familiar to those skilled in the art, are the equivalent of the filter unit I8 shown in Fig. 3.

For convenience, the batteries 52 and 54 have been shown as a source of plate and grid potenv tials for the various tubes constituting this receiver. It is understood, however, that in a commercial-form radio receiver these potentials would be supplied by the usual common powersupply system.

In Fig. a receiver similar to ,that shown in Fig. 4 is illustrated, similar parts being designated by the same reference characters. In this gure, two detectors I3 and I6' are provided. The detector |3 is for the purpose of detecting the audio-frequency signals andobtaining the automatic volume control effect and is connected in a manner similar to that in which detector I3 of Fig. 4 is connected, with the exception that the automatic volume control potentials are obtained from the single grid resistor 38, the biasing potentials on the two'ampliiiers I2v and 20 `being provided through the resistors M and l5,

respectively. In this connection also, the audio frequency supplied to the audio-frequency amplifier 5D is taken from an intermediate point on the resistor 38.

The auxiliary automatic tone control detector I6' is of the usual diode type and has its input supplied through connection 59 to the primary of the output transformer of the intermediate-frequency amplier 20. This connection includes the coupling condenser 6l).v A resistor 38' is included in the diode circuit and has its low-potential end connected to the grid of the tone control tube 29. 'Ihe tone control tube 29 and its circuit are the saine as in Fig. 4. Fig. 5 does not show a manual volume control, but may include Fig. 4 should it be so desired.

The usual cathode heating circuits are understood to be provided for the tubes shown in the circuits just described, the speciiic details of which constitute no part o1' the present invention.

What is claimed is:

1. In a radio receiver of the multi-stage vacum-tube type, a radio-frequency amplifying section, a detector, an audio-frequency amplifying section, a tone control circuit including a voltage responsive variable impedance device for cntrolling the fidelity of said audio-amplifying section, circuit means for causing said detector to develop audio-frequency voltages in accordance with the modulation of the carrier wave being received and a direct-current potential substantially proportional to the average amplitudeI of said carrier wave, means for applying said audiofrequency voltages to said audio-frequency amplifying section, and a plurality of means for ap.- plying said direct-current potential to said voltage responsive variable impedance device and to said radio-frequency amplifying section, whereby the delity and amplification of signals reproduced is controlledby the average amplitude of said carrier wave.

2. In a radio receiver of the multi-stage vacy num-tube type, a radio-frequency amplifying section, a detector, an audio-frequency amplifying section, a tone-control circuit including a `voltage responsive variable impedance device for signals reproduced is controlled by the average amplitude of said carrier wave, and supplemental means for' manually controlling the fidelity of the reproduced signal.

3. In a. radio receiver adapted for receiving a modulated carrier wave, the combination of a detector for providing a source of voltage compris'ing an audio-frequency voltage vvariable in accordance with the modulation of the received carrier wave and a unidirectional voltage directly proportional to the average amplitude of the carrier Wave supplied to said detector, means for sep- 40' arating said audio-frequency and said unidirectional voltages, means for utilizing said audiofrequency voltage to produce a signal, and a plurality of means for utilizing said unidirectional voltage separatelyto control the fidelity of said produced signal and to control the average amplitude of said supplied carrier'wave.

HAROLD MILLER LEWIS. 

